We have observed the nuclear magnetic resonance dispersion lineshape
in aluminum, under the common experimental condition of slow
passage with magnetic field modulation, while applying an rf field,
HI' intense enough to produce saturation, and have found that the
lineshape does not agree with the very weak HI theory of Bloembergen,
Purcell, and Pound, or with Redfield's strong HI theory. We have
qualitatively explained the lineshape by assuming that the spin system
in the rotating frame can be divided into two parts which have
unequal temperatures, and which are able to cross relax. For two
limiting cases, large HI' and HI small, but still large enough to
saturate, we have calculated the lineshape for all modulation
frequencies, and have obtained semiquantitative agreement. The small
HI calculation is based upon the theory of Provotorov and of Goldman.
The large HI theory is a phenomenological extension of Redfield's
theory. We have measured the dipole spin lattice relaxation time and
the cross relaxation time by observing the signal as a function of
modulation frequency. This method has the dual advantages of the
signal to noise of steady state resonance, and the measurement of time
on a frequency scale.